By Vitalii Hnidyi

KHARKIV, Ukraine, March 16 (Reuters) - In Kharkiv's regional infectious diseases hospital, doctors şişli escort those COVID-19 patients they can down to the bomb shelter in the basement when the air raid sirens sound.

But the most seriously ill, needing constant oxygen supply, cannot be moved, even if this means leaving them vulnerable to Russian bombardment.

"The ones in critical condition remain in their rooms. If we bring them down here they will simply die," said Pavlo Nartov, the hospital's director.

"Most of our patients are on oxygen supply all the time. They can't be cut off from the oxygen."

Kharkiv, Ukraine's second city in the east of the country, has been under Russian attack for weeks, leaving doctors with an impossible dilemma.

The regional emergency service said on Wednesday that at least 500 residents of the city have been killed since the start of Russia's invasion on Feb.

24.

Mayor Ihor Terekhov said in a televised interview on Tuesday that more than 600 buildings have been destroyed in Kharkiv, including schools, nurseries and hospitals.

"The Russian army is constantly shelling from the ground and the air," he said.

Nartov is relieved his hospital has been spared for now, but they are preparing for the worst.

"The situation is difficult and tense. As you can see, sick people, covered windows, bombardment going on from morning till night. Thank god, our territory, our hospital has not yet been hit," he said.

The staff are now learning how to use a gas mask, in case of a chemical attack.

Natalya Titarenko, who works at the hospital, said the building she lives in and the building of her sister had both been hit by Russian shells.

She recalled hearing a loud noise.

"My husband said 'they hit the house', there was a cloud of dust in the apartment and our neighbours started screaming. I opened the door, it was not damaged, I opened it and I saw in the yard a smoke curtain, dust, glass smashed everywhere, blown out doors."

"There was non-stop fighting for three days, vey loud."

Russia says it does not target civilians, describing its actions as a "special operation" to demilitarise and "de-Nazify" Ukraine.

Ukraine and Western allies call this a baseless pretext for Russia's invasion of the democratic country of 44 million. (Reporting by Vitalii Hnidyi Editing by Alexandra Hudson)